1. Field of the Invention
The present invention relates to reclosable plastic bags and packages of the type in which food products, such as chips and cereal, and other goods may be packaged for sale to consumers. More particularly, the present invention relates to improvements in the art of making reclosable plastic bags on form-fill-seal (FFS) machines wherein the zipper is applied transverse to the running direction of the film from which the bag is formed.
2. Description of the Prior Art
The present invention relates to improvements in the package-making art and may be practiced in the manufacture of thermoplastic bags and packages of the kind that may be used for various consumer products, but which are particularly useful for food products which must be kept in moisture and air-tight packages, free from leakage until initially opened for access to the product contents, which packages are then reclosable by zipper means to protect any remainder of the product therein.
The prior art is fairly well-developed, but nevertheless remains susceptible to improvement contributing to increased efficiency and cost effectiveness.
In particular, the present invention relates to the area of reclosable packaging known as the "transverse zipper". When making a bag with a transverse zipper, the zipper is attached transverse to the longitudinal axis of the material used to make the bag, as opposed to being attached to the bag material parallel to the longitudinal axis. A method and apparatus for making reclosable plastic bags with a transverse zipper is disclosed in U.S. Pat. No. 4,909,017.
A typical prior art FFS machine 10 configured for making transverse-zippered reclosable plastic bags is shown in FIG. 1. Thermoplastic film 12 from which the bags will be formed is paid off from a continuous roll thereof 14 in increments equal to the length of the bags which will ultimately be formed from the film 12 in the FFS machine 10. The longitudinal axis of the film 12 is parallel to the direction of travel of the film 12. Each time the film 12 comes to rest, a zipper strip 16 supplied from a continuous roll 24 is positioned on the film 12 transverse to the longitudinal axis by a positioning device 18, with one profile 20 of the zipper strip 16 on top of the other profile 22 of the zipper strip 16. The positioning device 18 can take any of a variety of forms well-known to those skilled in the reclosable packaging art, such as a vacuum conveyor for pulling the zipper strip 16 across the film 12 and a knife for cutting the zipper strip 16 from the continuous roll thereof 24.
The zipper strip 16 is then initially sealed or tacked to the thermoplastic film 12 by an attaching device 26, such as a pair of heater bars. FIG. 2 shows a cross section of the zipper strip 16 and the film 12 just after the zipper strip 16 has been attached thereto by the attaching device 26. Each profile has a leading flange 28, 29 projecting in the direction of motion of the film 12 and a trailing flange 30, 31 projecting in a direction opposite to its leading flange. The orientation of the profiles depends on which side of the zipper, if any, is particularly adapted to be the opening side. As disclosed in U.S. Pat. No. 4,909,017, the zipper strip 16 has a length approximately equal to half the width of the film 12 and is disposed centrally thereon. The profile flanges are substantially parallel to each other.
After the zipper strip 16 is positioned on the thermoplastic film 12, the attaching device 26 attaches the leading flange 29 of the lower profile 22 to the thermoplastic film 12. A series of zipper strips 16 are thusly initially attached to the thermoplastic film 12 at bag length increments as the thermoplastic film 12 is paid off the continuous roll thereof 14, as shown in FIG. 1. The transverse zipper-equipped film is then fed into the FFS machine 10, where the bags are formed and the final zipper seals are made.
At the FFS machine 10, the thermoplastic film 12 is fed downwardly over a forming collar 34 and folded around a filling tube 36. The edges of the film are brought together and pressed together by a pair of rollers 38. The edges are then sealed together by heater bars 40 to form a longitudinal back seal 42. Contents may then be dropped through the tube 36 into a bag 44 which has a lower seal 46. As discussed below, the lower seal 46 was made when the preceding bag was completed.
After introduction of the contents, the top of the bag is completed by the action of cross seal jaws 48, which perform five substantially simultaneous functions. First, the cross seal jaws 48 finally seal the leading flanges 28, 29 of the profiles to front and back bag walls without sealing the leading flanges 28, 29 to each other. Second, the cross seal jaws 48 seal the trailing flanges 30, 31 of the zipper strip to the front and back bag walls without sealing the trailing flanges 30, 31 to each other. The use of a heat activated adhesive on the flanges facilitates sealing of the flanges to the bag walls without sealing the flanges to each other. Third, the cross seal jaws 48 seal the top of the bag to form a pilfer evident seal 50. Fourth, the cross seal jaws 48 make the lower seal 46 for the succeeding bag. And fifth, the cross seal jaws 48 cut the completed bag 44 from the film 12.
Manufacture of transverse-zippered reclosable bags in accordance with the foregoing, however, has proven problematic. Specifically, because the leading flange 28 of the upper profile 20 is not initially attached to the thermoplastic film 12 by the attaching device 26, the upper leading flange 28 has a tendency to flare out as the film enters the FFS machine and is fed over the forming collar 34, as shown in FIG. 3. The result of this flaring is that the unsecured upper leading flange 28 has a tendency to either jam the FFS machine or to become caught on the machine and thereby cause the upper profile 20 to detach from the lower profile 22.
While the zipper strip 16 can be made without a leading flange 28 on the upper profile 20 in order to avoid this problem, the absence of the upper leading flange and its subsequent attachment to one of the bag walls in the FFS machine can result in poor zipper opening mechanics and a weak seal between the zipper strip and the completed bag. A four-flange seal, where all four zipper flanges are sealed to the bag, is the best and most secure method of attaching the zipper.